Transfusion therapy remains an important treatment modality for patients with sickle cell disease (SCD). However, some patients develop antibodies against the allogeneic transfused cells, causing major life- threatening complications for the patient. Understanding the immune triggers of alloimmunization and why some patients develop these antibodies is likely to lead to development of diagnostic as well as therapeutic interventions. T follicular helper (TFH) cells provide help to B cells in the generation of antibod responses as well as in production of long-lasting IgG antibodies and are likely to be involved in alloimmunization biology. Exiting new studies have found TFH related cells in the circulation in humans and mice expressing similar markers as lymphoid TFH cells (including CXCR5, ICOS, CD40L, IL-21) that can promote antibody production and their levels correlate with autoantibodies and levels of protective antibodies. Different circulating TFH subsets displaying TH1, TH2 and TH17 like effector functions have been identified. Intriguingly, TIGIT and PD-1 are also expressed on TFH cells, although their relationship to specific effector functions has not been described. These molecules are considered exhaustion markers or immune checkpoints expressed following chronic antigen stimulation and associated with low type I responses. Alloimmunization occurring after repeated transfusions could embody a similar model of repeated/chronic stimulation and be impacted by the expression of these 2 molecules on TFH cells. Our preliminary data indicate that TIGIT can indeed identify a subset of blood TFH cells with potent TFH functions and low TH1 like response as compared to TFH cells lacking TIGIT. These data suggest that upregulation of TIGIT (and PD-1) pathways on TFH cells may be directly involved in phenotype plasticity from differentiated CD4+ TFH cells displaying type 1 like properties toward strong B cell help. We further hypothesize that TIGIT (and PD-1) triggering on TFH cells is heightened in alloimmunized as compared to non-alloimmunized patients. To test these hypotheses, we will first examine the molecular regulation by TIGIT pathway on circulating TFH cells in driving robust TFH versus type-1 responses. We will then determine whether PD-1 on TIGIT-expressing TFH cells has overlapping or additive role to TIGIT in TFH function and polarization. Finally, we will determine whether TIGIT-dependent T FH functions differ in chronically transfused alloimmunized as compared to non-alloimmunized patients with SCD patients. Together, these data will determine whether alterations in TIGIT (and PD-1) pathway contributes to the plasticity of TFH effector functions and provide a better understanding of the TFH-related correlates of alloantibody response in SCD patients . Understanding the molecular mechanisms of how TFH cells drive alloimmunization in SCD m ay help toward future identification of biomarkers of alloimmunization and therapeutic strategies for its prevention in this vulnerable population.